Flexible printed circuit boards are used in electronic devices where the circuit is bent one time during installation, such as in the dashboard of an automobile, and/or where the circuit is flexed many times, such as in a camera, and/or where the circuit is flexed repeatedly, such as in a computer disk drive. Copper foil is an integral part of the flexible printed circuit board and it must therefore withstand the bending stress imposed upon it.
Electrodeposited copper foil may be used for flexible circuits. However, conventional electrodeposited copper foil has a fatigue ductility of about 20 to 50 percent. The fatigue ductility may be slightly improved if the electrodeposited copper foil is heat treated. Heat treated electrodeposited copper foil has a fatigue ductility level which is acceptable for single bends and large radius bends.
Conventional electrodeposited copper foil is made using an electrolyte containing about 30 to about 150 ppm free chloride ions. The free chloride ion, the presence of which may be attributable to water contamination or intentional addition, roughens the foil. As a result, the copper foil possesses high bond strength between itself and laminate resins because of the larger grain size and increased surface topography (more dendritic). This is because free chloride ions in the electrolyte contribute to a columnar grain structure in the resultant electrodeposited copper foil. However, copper foil having a columnar grain structure is not conducive to good flex performance because of failure at the large grain boundaries which are vertical relative to the foil plane. Moreover, conventional electrodeposited copper foils are produced using electrolytes containing organic additives such as animal glue. However, the presence of organic additives in the electrolyte results in copper foil which is resistant to metallurgical change upon heating and is weakened at the grain boundaries, which in turn results in a lower fatigue ductility. Accordingly, conventional electrodeposited copper foil and heat treated electrodeposited copper foil are not entirely appropriate for flex circuits.
In the hard-rolled state, wrought copper foil has sufficient strength to make it handleable. Wrought copper foils have a tendency to low temperature anneal. Heating wrought copper foil, for example by laminating copper foil to various polymeric resins, anneals the copper foil thus enhancing its fatigue ductility. Wrought copper foil is amenable to recrystallized grain growth during heat treatment at relatively low temperatures, about 200.degree. C. and less. Accordingly, wrought copper foil is currently preferred for higher performance dynamic flex applications, for instance flex circuits. Similarly, annealed wrought copper foil is also preferred for higher performance dynamic flex applications. Annealed wrought copper foil possesses a larger grain size and grain orientation which lead to better fatigue ductility compared to annealed electrodeposited copper foil. Annealed wrought copper foil has a fatigue ductility greater than the 65%, the minimum required in ANSI/IPC spec MF 150 F for Grade 7 copper foil. Wrought copper foil and annealed wrought copper foil, however, are subject to oxide inclusions which consequently degrade the properties and characteristics of the foil.
Generally speaking, the copper foils described above do not satisfy all the demands of copper foil for use in flex circuits. More specifically, electrodeposited copper foil and heat treated copper foil have low fatigue ductilities. Because of the grain structure and oxide inclusions of wrought copper foil, wrought copper foil has limited flex cycle performance. Furthermore, wrought copper foil exhibits anisotropic flex performance because of directional rolling. Annealed wrought copper foil has low tensile strength which leads to poor handleability. Wrought copper foil and annealed wrought copper foil have lower copper to laminate bond strength because of low surface roughness, limited thinness because of mechanical limitations associated with their manufacture, and limited width because of additional mechanical limitations associated with their manufacture.
U.S. Pat. No. 5,431,803 discloses a controlled low profile electrodeposited copper foil having a substantially uniform randomly oriented grain structure that is essentially columnar grain free and twin boundary free and having an average grain size of up to about 10 microns. The reference indicates that this foil has an ultimate tensile strength measured at 23.degree. C. in the range of about 87,000 to about 120,000 psi and an elongation measured at 180.degree. C. of about 15% to about 28%. The reference also discloses a process for making the foregoing foil, the process comprising: (A) flowing an electrolyte solution between an anode and a cathode and applying an effective amount of voltage across said anode and said cathode to deposit copper on said cathode; said electrolyte solution comprising copper ions, sulfate ions and at least one organic additive or derivative thereof, the chloride ion concentration of said solution being up to about 1 ppm; the current density being in the range of about 0.1 to about 5 A/cm.sup.2 ; and (B) removing copper foil from said cathode.